This invention relates to a boring apparatus in general, and, in particular, to an apparatus for enlarging the diameter of an existing bore within the interior of a turbine shaft or the like.
The existence of stress risers or stress points in the vicinity of the center line of turbine or other shafts used in high-power applications such as hydroelectric plants and the like has long presented a problem to engineers and manufacturers. These built-in stress risers and stress points are inclusions and impurities introduced into the shaft at the time it is manufactured or formed. Since it is desirable, and often necessary, to eliminate or at least reduce the number of stress risers or stress points formed in the shafts, an attempt has been made to accomplish this by drilling a three to four inch diameter hole along the central axis of the turbine shaft. This hole, in addition to removing all the material along its length would also remove these inclusions.
Such a procedure was considered to be adequate until recently when the fast-developing field of nuclear density testing was applied to testing the soundness of such drilled turbine shafts. Unfortunately, it was discovered that a number of stress points usually remained in the shafts close to the center hole, despite the opening of the three or four inch bore.
It is not yet known with certainty in the industry whether these small inclusions or cracks propagate with time, as expected, because of the lack of previous test data. However, because of the expense and potential danger involved in failure of a turbine shaft while in operation in a large scale utility or the like, it has been considered extremely desirable to eliminate, if possible, the number of these stress points. The problem with so doing would be that boring a larger diameter hole uniformly from one end of the shaft to another end of the shaft would severely reduce the strength of the shaft at the ends. It would be ideal, therefore, to "bottle bore" the shaft, that is, to enlarge an existing three or four inch bore to a larger diameter along a selected portion of the interior length of the shaft, while tapering back to the diameter of the existing bore near each end. There is no presently existing equipment in the industry, however, which can accomplish this type of bottle bore to the dimensions necessary. No prior apparatus or method will allow expansion of the interior diameter to more than twice the width of the tool head. Moreover, prior apparatus, such as lathes, are not portable and require turning of the entire turbine to accomplish boring.